The present invention relates generally to insulated molds and more particularly to a mold insert useful in a process for molding optical and compact disks having reduced birefringence, improved pits replication and improved molding characteristics.
Various types of molds have long been in use for preparing shaped articles from thermoplastic resins. Molds for these purposes are typically manufactured from metal or a similar material having high thermal conductivity. For most purposes, high thermal conductivity is desirable since it permits the resin in the mold to cool rapidly, shortening the molding cycle time. At times, however, cooling is so rapid that the resin freezes instantaneously at the mold surface upon introduction into the mold, forming a thin solid layer which, especially if it contains a filler, can create rough surfaces, voids, porosity and high levels of residual stress and orientation. In an optical disk, such imperfections impede the optical properties and decrease or eliminate performance.
There have recently been disclosed multilayer molds in which a metal core has an insulating layer bonded thereto for the purpose of slowing the initial cooling of the resin during the molding operation. The insulating layer is fabricated of material having low thermal conductivity, thus slowing the cooling of the molded resin, and also having good resistance to high temperature degradation, permitting use in a mold maintained at high temperatures. In order to improve durability of the mold and improve surface qualify one or more skin layers of hard material, typically metal, is bonded to the insulating layer. The skin layer may be deposited by such operations as electroless deposition, electrolytic deposition and combinations thereof. Due to the insulation, the skin layer retains heat during the molding operation, thereby avoiding the surface irregularities created by rapid surface cooling. Thus, these devices provide a surface with lower residual stress and less orientation than a conventionally molded part while maintaining a relatively short cycle time.
Depending on specific requirements, plastic parts can be made by any of a number of known molding processes such as blow molding, compression injection molding, molding and injection compression molding.
In compression molding, composite blanks of glass reinforced thermoplastic sheets are heated. The material is heated above its melting point or if an amorphous material at least substantially above its glass transition temperature. When the composite blanks are heated, they expand (loft) due to the recoil forces within the fibers. The hot blanks are then pressed between cool mold surfaces which are below the melting point or glass transition temperature. Contact with the cool mold surfaces results in frozen resin on the surface of the blank. This creates unfilled areas in the form of exposed fibers and surface porosity. The resin at the cold surface is frozen and does not flow. Thus, rough boundaries between the charged and newly formed areas are produced.
Injection molding involves injecting molten thermoplastic resin into a mold apparatus. Molds for injection molding of thermoplastic resin are usually made from metal material such as iron, steel, stainless steel, aluminum alloy or brass. Such materials are advantageous in that they have high thermal conductivity and thus allow the melt of thermoplastic resin to cool rapidly and shorten the molding cycle time. However, because of the rapid cooling, the injected resin freezes instantaneously at the mold surface, resulting in a thin solid layer. Quick quenching of the melt at the mold surface creates several problems. The freezing of these materials at the mold surfaces creates rough surfaces. Processing difficulties arise especially when producing thin parts requiring a high quality optical surface. The quick solidification of the melt combined with, for example, variable radial flowability of the materials makes it difficult to achieve the kind of uniform melt flow required for an optical disk. This is important when considering the quality of pits replication required for optical disks. Non-uniform flow can result in bad areas with high bit errors. The use of multiple gates is not generally thought to be a practical expedient to remedy non-uniform melt flow in an optical medium, because weld lines are produced which can cause optical flaws.
In injection compression molding which is a combined process, a hot thermoplastic melt is injected into a mold cavity. The parting line of the mold is positioned open or allowed to be forced open by the injected melt typically 0.05" to 0.3" inches. The clamping force is increased initiating the compression stroke of the mold forcing the melt to fill the cavity. In many instances the velocity of the melt front through the cavity changes as the injection stroke stops and the compression stroke begins. This distinct change in melt front velocity is often characterized by a stall followed by a surge in the melt front.
The problems associated with injection molding are likewise associated with injection compression molding. Another important issue to be considered in molding of high quality parts is the residual stress in such molded parts. Residual stress can result in dimensional instability and non-uniform birefringence. Dimensional stability and uniformity of birefringence are critically required for applications such as the manufacture of optical disks. For example, dimensional instability can result in differential expansion and contraction, which in turn can cause unacceptable variations in concentricity, eccentricity flatness of the medium.
Birefringence mechanisms, (i.e., retardation and the influence of molding and specific process conditions on residual retardation or optical path difference) are also crucial factors to be considered in connection with optical disk manufacture.
Retardation .GAMMA. is defined as: EQU .GAMMA.-R.lambda. (1)
Where R is phase retardation and .lambda. is the wave length of the source.
Birefringence, .DELTA.n, is then defined as: ##EQU1##
Where t is the thickness of the optical medium. Thus, birefringence is a dimensionless quantity.
Birefringence is a net effect through a sample, which is predominately molten at the cessation of flow. Thus, molecular orientation in the quenched skins and slowly cooled core have a direct effect on the retardation. Molecular orientation is proportional to the applied stress field creating the flow which is related to birefringence, according to the following expression: EQU .DELTA.n.sub.13 -(n.sub.11 -n.sub.33)-C(.sigma..sub.11 -.sigma..sub.33)(3)
for a simple shear flow where C is the stress-optical coefficient. This analysis can be taken a step further by relating the normal stress difference (.sigma..sub.11 -.sigma..sub.33) to the shear stress, .sigma..sub.12, as follows: EQU (.sigma..sub.11 -.sigma..sub.33).alpha..sigma..sup.2.sub.12( 4)
Hence, substituting equation (4) into equation (3) gives EQU (n.sub.11 -n.sub.33)-K.sigma..sup.2.sub.12 ( 5)
where K is a constant and n.sub.11 and n.sub.33 are the refractive indices in the flow and cross-flow directions, respectively. The expression is valid for polystyrene melts and for low molecular weight polycarbonate. In an optical disk, the flow originates at the center of the disk and radially diverges towards the outer edges as the melt fills the cavity. Hence, rendering the birefringence profile uniform in a diverging radial flow field is not a simple task. Molecular orientation varies radially as the flow front speed (and wall shear stress) decreases.
The phase retardation, .GAMMA., is usually expressed in terms of nanometers (10.sup.-9 meters). Since CD's are nominally 1.2 mm thick, retardation is typically specified, rather than birefringence, and retardation will be used where appropriate in the discussion below.
Retardation may be measured with a commercial instrument marketed by Hinds International. The instrument is a system consisting of a 2 mW He--Ne laser (.lambda.-632.8 nm), photoelastic modulator and lock-in amplifier. The output is stored in a Nicolet storage scope and then may be transferred to a floppy disk or x-y plotter. The wavelength of the He--Ne laser in the analyzer is 632.8 nm. This may be adjusted to .lambda.-780 nm as a reference and the output multiplied by 2 for double pass values.
A typical retardation profile is minimal at the hub and outer edge and is maximum at the midpoint of the annular disk area. The difference between the maximum and minimum is .DELTA..GAMMA.. To measure the profile, the disk is rotated in a direction perpendicular to the incident light which passes along a radial path from the outer edge to the hub. The rotating disk is then withdrawn along the same path resulting in a second measurement on the same disk. The maximum and minimum retardation may then be noted as well as the absolute difference .DELTA..GAMMA.. The extreme ends, i.e., outer edge and hub, of the retardation signal may be ignored because they do not contain recorded information.
The definitive test of a compact disk is the audio quality when played by a CD player. Assuming good aluminum film deposition and a good stamper, the accuracy of the encoded digital information is a function of the optical properties of the substrate and the pits replication of the stamper from which it is pressed.
Requirements for optical storage media are much more stringent than those specified for CD's. Normal retardation is reduced to .+-.20 nm and off-axis retardation (30.degree. off normal) must be below .+-.70 nm. A good birefringence profile is nearly isotropic. Both normal and off-axis measurements have relatively low, acceptable levels of retardation. In addition, the optical properties at each radial position should be as uniform as possible.
The interrelationship of process conditions, birefringence and pit replication is highly complex when manufacturing digital audio disks. The retardation profiles are a reliable measure of the effect which process conditions have on final optical properties. Circumferential variations reflect non-uniform heat transfer in the mold. Also, because the polycarbonate must cool against the nickel stamper with precise molding of the pits, heat transfer, here too, is important. Thus, improvement is required to render the heat transfer more uniform or at least more symmetrical about the central sprue.